1. Field of the Invention
The present invention relates to a radar imaging system and method for real-time or near real-time video imaging. In particular although not exclusively the present invention relates to a radar imaging system and method, for security screening and the like.
2. Discussion of the Background Art
Persons entering highly secure areas are often required to undergo some form of screening to ensure weapons or explosives are not being carried. These security concerns have led to the development of various techniques designed to discover undesirable objects hidden under a persons clothing.
One common process used to screen people entering a secure which has been used to good effect is the pat-down technique. Literally this requires security personnel to pat-down by hand the body of the person of interest through their clothing. This process can be quite time consuming where large numbers are to be screened, and the body contact can be considered some people to be both intrusive and offensive. Another common screening technique involves the use of hand held or walk-through metal detectors. Essentially these detectors rely on measuring induced currents in any metal object within range which is typically very short. Thus such detectors are not readily suited to applications where screening at a distance is desirable. Furthermore the detectors are only cable of detecting metal objects and fail to detect objects composed of ceramics e.g. ceramic knives, plastics or the like which can be a security concern.
It is generally known that electromagnetic (em) radiation in the millimetre wavelength of the spectrum readily penetrates through clothing. Likewise both microwave and x-rays also readily penetrate through articles of clothing as well as dust and smoke. X-rays have proved effective but are not acceptable in public places because of their inherent radiation hazard. Low-level electromagnetic radiation emitted in the millimetre and microwave wavelengths of the spectrum poses no known radiation hazard and is widely accepted for mobile phone and wireless applications. As such these wavelengths are well suited for use in security imaging applications.
One technique that makes use of the fact that electromagnetic radiation penetrates clothing is based on the use of radiometers which image the natural electromagnetic radiation from the body. Differing surface temperatures, their emissivity, and the differing scatter from background electromagnetic radiation provide contrast which can be focussed to form an image. Typically a radiometer will detect electromagnetic radiation in the mm wavelength bands. A millimetre wave radiometer will typically use a fast mechanically scanned reflector antenna. These mechanical scanning systems are unable to obtain fine image resolution at distant ranges. This is because a relatively larger scanner is required and it is not feasible to scan these large antennas at the required speed. Mechanically scanned systems also have a problem with achieving a fast enough refresh rate to capture images of a walking person. Another problem with mechanically scanned antennas is that they only focus at one distance and with a narrow depth of focus. This requires the person being screened to remain stationary at the focal point in front of the scanner for the duration of the scan. Yet another problem with radiometer systems is their susceptibility to the natural sources of radiation which can be very variable and affect the image contrast.
Radar imaging systems provide a more consistent illumination of the object to be imaged and phased array radar technology has the potential to provide images with multiple depths of focus, at longer ranges, and with fast refresh times. However a conventional phased array requires the whole aperture to be filled with transmitter and receiver mm wave elements, leading to an excessive cost.
Alternatives to the conventional phased array have been described in the literature, especially systems for imaging in the near proximity to the antenna array with a sparse array system.
An early publication “Synthetic Aperture Pulse Echo imaging with a Rectangular Boundary Array” by Kozick, R J and Kassam. S A, in IEEE Transactions on Image Processing Vol 2 No 1 January 1993, describes the basic principles of pulse-echo imaging at finite ranges with an array of the form described in our patent application. Their paper describes the process of imaging with pulses emitted for each transmitter/receiver pair combination in turn for a rectangular array formed with a pair of parallel transmitter elements and a pair of parallel receiver elements. Whilst this describes a process for imaging in the near field, their approach is not suited to applications where objects being imaged might move during the process of collecting data from all the pair combinations in turn.
A recent International patent application WO 2007/045026 describes a Synthetic Aperture Perimeter Array formed using the principles as described by Hoctor and Kassam (Proc IEEE Vol 78, No 4 April 1990, p 735-752). This reference describes a rectangular boundary aperture using one pair of sides for transmission and the other pair for reception. In order to collect the data required to form the image a radar waveform generator is switched in sequence to the elements of the transmitter array whilst a radar receiver system is sequentially switched to each receiver element in turn. The application shows how the process synthesises virtual elements located midway between each transmitter/receiver pair which can be processed in the manner of a filled array for imaging a distant scene. However the technique of synthesising a filed array in this manner is only applicable to the far field and will not focus in the near field. The technique described in WO 2007/045026 is not able to image a walking person for security screening applications. This is because typical walking movements will change signal phases over the time required to collect data from all transmitter and receiver combinations. This corrupts the focussing process. In contrast the technique described in our invention overcomes or at least ameliorates this problem.
The array described in WO 2007/045026 is primarily focused in the far field. For security scanning and other such applications the array must be focussed in the near field. WO 2007/045026 provides no teaching as to how this may be accomplished with an array of this type. This process of synthesising virtual elements and using these for imaging is not valid in the near field. The notion of synthesised virtual elements is only valid for imaging very distant scenes.
Clearly it would be advantageous to provide a radar based imaging system that provides a relatively high refresh rate and which is capable of focussing on all distance, whilst achieving fine resolution in the vicinity of the radar and at longer distances.